32 research outputs found
Підвищення ефективності сепарації пилу у вихрових апаратах із зустрічними закрученими потоками із циліндричною сепараційною камерою (ВАЗЗПЦ) для підприємств хімічних та будівельних матеріалів
Актуальною проблемою, яка постає сьогодні перед вітчизняною
промисловістю, є вдосконалення техніки і технології охорони навколишнього
середовища в цілому, і, зокрема, зменшення рівня запиленості атмосферного
повітря.Вирішення проблеми міститься у процесі сепарації пилу у ВАЗЗПЦ і
безперервним вивантаженням уловлюваного пилу стосовно хімічних та
будівельних матеріалів
Mononuclear Rhodium(II) and Iridium(II) Complexes Supported by Tetradentate Pyridinophane Ligands
The
tetradentate ligands <i>N</i>,<i>N</i>′-dialkyl-2,11-diaza[3,3](2,6)pyridinophane
(<sup>R</sup>N4, where R = Me or <i>t</i>Bu) were employed
to synthesize and fully characterize [(<sup>R</sup>N4)M<sup>I</sup>(COD)]<sup>+</sup> complexes (M = Rh or Ir; COD = cyclooctadiene).
Interestingly, these complexes exhibit accessible oxidation potentials
and can generate detectable [(<sup>R</sup>N4)M<sup>II</sup>(COD)]<sup>2+</sup> complexes, which were characterized by electron paramagnetic
resonance and high-resolution electrospray ionization mass spectrometry.
Moreover, a rare mononuclear [(<sup>Me</sup>N4)Rh<sup>II</sup>(COD)]<sup>2+</sup> complex was isolated and crystallographically characterized,
allowing for a direct comparison with its rhodium(I) analogue. The
detailed characterization of such paramagnetic rhodium(II) and iridium(II)
complexes enables further investigation of their redox reactivity
The Aerobic Oxidation of a Pd(II) Dimethyl Complex Leads to Selective Ethane Elimination from a Pd(III) Intermediate
Oxidation of the Pd<sup>II</sup> complex (N4)Pd<sup>II</sup>Me<sub>2</sub> (N4 = <i>N</i>,<i>N</i>′-di-<i>tert</i>-butyl-2,11-diaza[3.3](2,6)pyridinophane) with O<sub>2</sub> or ROOH (R = H, <i>tert</i>-butyl, cumyl) produces
the Pd<sup>III</sup> species [(N4)Pd<sup>III</sup>Me<sub>2</sub>]<sup>+</sup>, followed by selective formation of ethane and the monomethyl
complex (N4)Pd<sup>II</sup>Me(OH). Cyclic voltammetry studies and
use of 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (DMPO) as
a spin trap suggest an inner-sphere mechanism for (N4)Pd<sup>II</sup>Me<sub>2</sub> oxidation by O<sub>2</sub> to generate a Pd<sup>III</sup>-superoxide intermediate. In addition, reaction of (N4)Pd<sup>II</sup>Me<sub>2</sub> with cumene hydroperoxide involves a heterolytic O–O
bond cleavage, implying a two-electron oxidation of the Pd<sup>II</sup> precursor and formation of a transient Pd<sup>IV</sup> intermediate.
Mechanistic studies of the C–C bond formation steps and crossover
experiments are consistent with a nonradical mechanism that involves
methyl group transfer and transient formation of a Pd<sup>IV</sup> species. Moreover, the (N4)Pd<sup>II</sup>Me(OH) complex formed
upon ethane elimination reacts with weakly acidic C–H bonds
of acetone and terminal alkynes, leading to formation of a new Pd<sup>II</sup>–C bond. Overall, this study represents the first
example of C–C bond formation upon aerobic oxidation of a Pd<sup>II</sup> dimethyl complex, with implications in the development of
Pd catalysts for aerobic oxidative coupling of C–H bonds
The Conformational Flexibility of the Tetradentate Ligand <sup>tBu</sup>N4 is Essential for the Stabilization of (<sup>tBu</sup>N4)Pd<sup>III</sup> Complexes
The
conformationally flexible tetradentate pyridinophane ligand <sup>tBu</sup>N4 effectively lowers the oxidation potential of (<sup>tBu</sup>N4)Pd<sup>II</sup> complexes and promotes their facile chemical and electrochemical
oxidation, including unpredecented aerobic oxidation reactivity. While
the low potential of a number of Pd<sup>II</sup> (and Pt<sup>II</sup>) complexes supported by various <i>fac</i>-chelating polydentate
ligands is often attributed to the presence of a coordinating group
in the axial position of the metal center, no detailed electrochemical
studies have been reported for such systems. Described herein is the
detailed electrochemical investigation of the effect of ligand conformation
on the redox properties of the corresponding Pd<sup>II</sup> complexes.
These Pd complexes adopt different conformations in solution, as supported
by studies using variable scan rate, variable-temperature cyclic voltammetry
(CV), differential pulse voltammety, and digital CV simulations at
variable scan rates. The effect of the axial amine protonation on
the spectroscopic and electrochemical properties of the complexes
was also investigated. A number of new Pd<sup>III</sup> complexes
were characterized by electron paramagnetic resonance, UV–vis
spectroscopy, and X-ray diffraction including [(<sup>tBu</sup>N4)Pd<sup>III</sup>Cl<sub>2</sub>]ClO<sub>4</sub>, a dicationic [(<sup>tBu</sup>N4)Pd<sup>III</sup>Me(MeCN)](OTf)<sub>2</sub>, and an unstable
tricationic [(<sup>tBu</sup>N4)Pd<sup>III</sup>(EtCN)<sub>2</sub>]<sup>3+</sup> species. Although the electron-rich neutral complexes
(<sup>tBu</sup>N4)PdMeCl and (<sup>tBu</sup>N4)PdMe<sub>2</sub> are
present in solution as a single isomer with the axial amines not interacting
with the metal center, their low oxidation potentials are due to the
presence of a minor conformer in which the <sup>tBu</sup>N4 ligand
adopts a tridentade (κ<sup>3</sup>) conformation. In addition,
the redox properties of the (<sup>tBu</sup>N4)Pd complexes show a
significant temperature dependence, as the low-temperature behavior
is mainly due to the contribution from the major, most stable conformer,
while the room-temperature redox properties are due to the formation
of the minor, more easily oxidized conformer(s) with the <sup>tBu</sup>N4 ligand acting as a tridentate (κ<sup>3</sup>) or tetradentate
(κ<sup>4</sup>) ligand. Overall, the coordination to the metal
center of each axial amine donor of the <sup>tBu</sup>N4 ligand leads
to a lowering of the Pd<sup>II/III</sup> oxidation potential by ∼0.6
V. These detailed electrochemical studies can thus provide important
insights into the design of new ligands that can promote Pd-catalyzed
oxidation reactions employing mild oxidants such as O<sub>2</sub>
Oxidative C–C Bond Formation Reactivity of Organometallic Ni(II), Ni(III), and Ni(IV) Complexes
The
use of the tridentate ligand 1,4,7-trimethyl-1,4,7-triazacyclononane
(Me<sub>3</sub>tacn) and the cyclic alkyl/aryl C-donor ligand -CH<sub>2</sub>CMe<sub>2</sub>-<i>o</i>-C<sub>6</sub>H<sub>4</sub>- (cycloneophyl) allows for the synthesis of isolable organometallic
Ni<sup>II</sup>, Ni<sup>III</sup>, and Ni<sup>IV</sup> complexes.
Surprisingly, the five-coordinate Ni<sup>III</sup> complex is stable
both in solution and the solid state, and exhibits limited C-C bond
formation reactivity. Oxidation by one electron of this Ni<sup>III</sup> species generates a six-coordinate Ni<sup>IV</sup> complex, with
an acetonitrile molecule bound to Ni. Interestingly, illumination
of the Ni<sup>IV</sup> complex with blue LEDs results in rapid formation
of the cyclic C-C product at room temperature. This reactivity has
important implications for the recently developed dual Ni/photoredox
catalytic systems proposed to involve high-valent organometallic Ni
intermediates. Additional reactivity studies show the corresponding
Ni<sup>II</sup> species undergoes oxidative addition with alkyl halides,
as well as rapid oxidation by O<sub>2</sub>, to generate detectable
Ni<sup>III</sup> and/or Ni<sup>IV</sup> intermediates and followed
by C-C bond formation
Diastereoselective Attack on Chiral-at-Metal Ruthenium Allenylidene Complexes To Give Alkynyl Complexes
New
chiral ruthenium(II) allenylidene complexes were synthesized,
and their reactivity with nucleophiles to give alkynyl complexes was
investigated. The new allenylidene complex (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>){CCC(<i>t</i>-Bu)(2-naphthyl)}]<sup>+</sup>PF<sub>6</sub><sup>–</sup> was synthesized from the chloro precursor complex (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[RuCl(Ind)(PPh<sub>3</sub>)(<b>6</b>)] and the racemic propargylic alcohol HCCC(OH)(<i>t</i>-Bu)(2-naphthyl) and obtained in 96% yield, where
(<i>R</i><sub>ax</sub>)-<b>6</b> is a chiral phosphoramidite
and Ind an anionic indenyl ligand. The precursor and the allenylidene
complex are chiral-at-metal, and the chiral information is completely
transferred from the chloro precursor to the product allenylidene
complex, both of which show the same absolute configuration, as demonstrated
by X-ray diffraction. Together with the known allenylidene complex
(<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>)(CCCPh<sub>2</sub>)]<sup>+</sup>PF<sub>6</sub><sup>–</sup>, the attack
of <i>n</i>-BuLi, MeLi, LiCCPh, and lithium 1-phenylethenolate
nucleophiles on the allenylidene chain of (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>){CCC(<i>t</i>-Bu)(2-naphthyl)}]<sup>+</sup>PF<sub>6</sub><sup>–</sup> was investigated. The nucleophiles (Nu) reacted selectively with
the gamma carbon of the allenylidene complexes to give the alkynyl
complexes (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>)(CC–CPh<sub>2</sub>Nu)] and (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>){CC–C(<i>t</i>-Bu)(2-naphthyl)Nu}] in 40% to 96% isolated yields.
In the case of (<i>R</i><sub>Ru</sub>,<i>R</i><sub>ax</sub>)-[Ru(Ind)(PPh<sub>3</sub>)(<b>6</b>){CC–C*(<i>t</i>-Bu)(2-naphthyl)Nu}], the gamma carbon C* becomes
stereogenic upon attack of the nucleophiles. As assessed by <sup>31</sup>P{<sup>1</sup>H} NMR, diastereodifferentiation took place, and the
alkynyl complexes were isolated as diastereomeric mixtures with diastereomeric
ratios between 60:40 and 84:16. The diastereodifferentiation originated
only from the stereogenic metal center and the monodentate, chiral
ligand. The study allows for investigation of stereoselective, nucleophilic
attack of allenylidene complexes to give optically active, quaternary
alkynes, which play a role in potential catalytic versions of nucleophilic
substitution reactions of propargylic alcohols
Synthesis of a Smoothened Cholesterol: 18,19-Di-nor-cholesterol
Herein,
we report the first synthesis of a demethylated form of
cholesterol (18,19-di-nor-cholesterol), in which the C18 and C19 methyl
groups of the β-face were eliminated. Recent molecular simulations
modeling 18,19-di-nor-cholesterol have suggested that cholesterol’s
opposing rough β-face and smooth α-face play necessary
roles in cholesterol’s membrane condensing abilities and, additionally,
that specific facial preferences are displayed as cholesterol interacts
with different neighboring lipids and transmembrane proteins. Inspired
by these poorly characterized biochemical interactions, an extensive
18-step synthesis was completed as part of a collaborative effort,
wherein synthesizing a “smoothened” cholesterol analogue
would provide a direct way to experimentally measure the significance
of the β-face methyl groups. Starting from known perhydrochrysenone <b>A</b>, the synthesis of 18,19-di-nor-cholesterol was accomplished
with an excellent overall yield of 3.5%. The use of the highly stereoselective
Dieckmann condensation and the employment of Evans’ chiral
auxiliary were both key to ensuring the success of this synthesis
The Aerobic Oxidation of a Pd(II) Dimethyl Complex Leads to Selective Ethane Elimination from a Pd(III) Intermediate
Oxidation of the Pd<sup>II</sup> complex (N4)Pd<sup>II</sup>Me<sub>2</sub> (N4 = <i>N</i>,<i>N</i>′-di-<i>tert</i>-butyl-2,11-diaza[3.3](2,6)pyridinophane) with O<sub>2</sub> or ROOH (R = H, <i>tert</i>-butyl, cumyl) produces
the Pd<sup>III</sup> species [(N4)Pd<sup>III</sup>Me<sub>2</sub>]<sup>+</sup>, followed by selective formation of ethane and the monomethyl
complex (N4)Pd<sup>II</sup>Me(OH). Cyclic voltammetry studies and
use of 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (DMPO) as
a spin trap suggest an inner-sphere mechanism for (N4)Pd<sup>II</sup>Me<sub>2</sub> oxidation by O<sub>2</sub> to generate a Pd<sup>III</sup>-superoxide intermediate. In addition, reaction of (N4)Pd<sup>II</sup>Me<sub>2</sub> with cumene hydroperoxide involves a heterolytic O–O
bond cleavage, implying a two-electron oxidation of the Pd<sup>II</sup> precursor and formation of a transient Pd<sup>IV</sup> intermediate.
Mechanistic studies of the C–C bond formation steps and crossover
experiments are consistent with a nonradical mechanism that involves
methyl group transfer and transient formation of a Pd<sup>IV</sup> species. Moreover, the (N4)Pd<sup>II</sup>Me(OH) complex formed
upon ethane elimination reacts with weakly acidic C–H bonds
of acetone and terminal alkynes, leading to formation of a new Pd<sup>II</sup>–C bond. Overall, this study represents the first
example of C–C bond formation upon aerobic oxidation of a Pd<sup>II</sup> dimethyl complex, with implications in the development of
Pd catalysts for aerobic oxidative coupling of C–H bonds
Aromatic Methoxylation and Hydroxylation by Organometallic High-Valent Nickel Complexes
Herein we report the synthesis and
reactivity of several organometallic
Ni<sup>III</sup> complexes stabilized by a modified tetradentate pyridinophane
ligand containing one phenyl group. A room temperature stable dicationic
Ni<sup>III</sup>–disolvento complex was also isolated, and
the presence of two available <i>cis</i> coordination sites
in this complex offers an opportunity to probe the C-heteroatom bond
formation reactivity of high-valent Ni centers. Interestingly, the
Ni<sup>III</sup>-dihydroxide and Ni<sup>III</sup>-dimethoxide species
can be synthesized, and they undergo aryl methoxylation and hydroxylation
that is favored by addition of oxidant, which also limits the β-hydride
elimination side reaction. Overall, these results provide strong evidence
for the involvement of high-valent organometallic Ni species, possibly
both Ni<sup>III</sup> and Ni<sup>IV</sup> species, in oxidatively
induced C-heteroatom bond formation reactions
Late First-Row Transition Metal Complexes of a Tetradentate Pyridinophane Ligand: Electronic Properties and Reactivity Implications
The
synthesis and structural comparison are reported herein for a series
of late first-row transition metal complexes using a macrocyclic pyridinophane
ligand, <i>N</i>,<i>N</i>′-di-<i>tert</i>-butyl-2,11-diaza[3.3](2,6)pyridinophane (<sup>tBu</sup>N4). The <sup>tBu</sup>N4 ligand enforces a distorted octahedral geometry in complexes
[(<sup>tBu</sup>N4)M<sup>II</sup>(MeCN)<sub>2</sub>](OTf)<sub>2</sub> (M = Fe<sup>II</sup>, Co<sup>II</sup>, Ni<sup>II</sup>, Cu<sup>II</sup>), [(<sup>tBu</sup>N4)Zn<sup>II</sup>(MeCN)(OTf)](OTf), and [(<sup>tBu</sup>N4)Fe<sup>III</sup>(OMe)<sub>2</sub>](OTf), with elongated
axial M–N<sub>amine</sub> distances compared to the equatorial
M–N<sub>py</sub> distances. The geometry of [(<sup>tBu</sup>N4)Cu<sup>I</sup>(MeCN)](OTf) is pentacoordinate with weak axial
interactions with the amine N-donors of <sup>tBu</sup>N4. Complexes
[(<sup>tBu</sup>N4)M(MeCN)<sub>2</sub>](OTf)<sub>2</sub> (M = Fe,
Co) exhibit magnetic properties that are intermediate between those
expected for high spin and low spin complexes. Electrochemical studies
of (<sup>tBu</sup>N4)M complexes suggest that <sup>tBu</sup>N4 is
suitable to stabilize Co<sup>I</sup>, Ni<sup>I</sup>, Co<sup>III</sup>, Fe<sup>III</sup> solvato-complexes, while the electrochemical oxidation
of (<sup>tBu</sup>N4)NiCl<sub>2</sub> complex leads to formation of
a Ni<sup>III</sup> species, supporting the ability of the <sup>tBu</sup>N4 ligand to stabilize first row transition metal complexes in various
oxidation states. Importantly, the [(<sup>tBu</sup>N4)M<sup>II</sup>(MeCN)<sub>2</sub>]<sup>2+</sup> complexes exhibit two available <i>cis</i> coordination sites and thus can mediate reactions involving
exogenous ligands. For example, the [(<sup>tBu</sup>N4)Cu<sup>II</sup>(MeCN)<sub>2</sub>]<sup>2+</sup> species acts as an efficient Lewis
acid and promotes an uncommon hydrolytic coupling of nitriles. In
addition, initial UV–vis and electron paramagnetic resonance
(EPR) studies show that the [(<sup>tBu</sup>N4)Fe<sup>II</sup>(MeCN)<sub>2</sub>]<sup>2+</sup> complex reacts with oxidants such as H<sub>2</sub>O<sub>2</sub> and peracetic acid to form high-valent Fe transient
species. Overall, these results suggest that the (<sup>tBu</sup>N4)M<sup>II</sup> systems should be able to promote redox transformations
involving exogenous substrates